Draft: Add modifiable key bindings

Related to #146

This works, but there are a few issues to consider:

• One will have to do to_lowercase() to get a lower case character
  • Change all key bindings to uppercase? This seems to be the standard.
• There is a KeyboardShortcut enum in egui which would some the issue to define ones own modifier keys etc. However, it is not deserializable, so one cannot easily modify it as part of a config. It will also require a rewrite of the key handling (if I am not wrong).